MRI systems utilize RF coils to transmit RF signals into selected portions of a body to excite magnetically aligned nuclei within that body. MRI devices typically employ at least one RF coil for transmitting the RF signals and either the same or another RF coil for receiving RF signals generated by the excited nuclei in the body. As may be expected, the switching of the RF transmit and receive coils are carefully controlled by a computer electrically connected to the respective coils (and the components associated with the coils, such as matching circuits and detuners) by control lines.
The control lines connecting the RF coils (and their associated components) to the computer are often a major source of RF coil system instability. This is caused by spurious RF currents carried along the surface of the RF shielding jacket associated with the control lines. Specifically, the cables that carry the control signals for the RF coils are generally surrounded by a conductive tubing to prevent RF signals from interfering with the signals on the control lines (and vice versa). The shielding jacket is grounded on a common ground to the coils and the computer controller. Thus, during RF transmissions, some of the RF energy may travel through a ground loop beginning in the control line shieldings and emerging in the digital and analog control circuits of the computer. This spurious RF energy on the ground lines of the control circuits can cause control circuit malfunctions, the production of spurious analog signals, degradation in the RF transmitter coil Q. Similarly, the spurious RF energy can cause receiver coil Q degradation and reduced isolation between the receiver coils.
While problems in maintaining isolation of the RF coils can be caused by coil-to-coil coupling, the present invention relates more particularly to conductive coupling. Coil-to-coil coupling is caused by the electrical and magnetic fields which the RF coils are designed to produce. Some of this coupling between the coils can be reduced by placing a number of capacitors in series with the coil traces, by placing some of the coils orthogonal to one another, or by allowing part of the coil traces to overlap slightly. Further reduction in this coupling can be achieved by using a low input impedance preamplifier as a part of a matching circuit to form an electrical trap circuit which would reduce coupling between the receiver coils. Conductive coupling, on the other hand, may not respond to even carefully executed decoupling strategies.
This conductive coupling can be caused by interconnecting control cables. Because the cables not only carry control signals but may also carry a spurious RF current, coupling between the RF coil and the connecting cable interface (conductive coupling) can occur. During the transmission period of an MRI session, if sufficient RF energy finds its way into the digital and analog circuits via the control signal lines, a control circuit malfunction and spurious analog signals can be produced. The coupling can also degrade receiver coil Q and coil-to-coil isolation.
The RF current on the control lines themselves can be effectively blocked by adding LC or RC filters in series with the control lines. However, physically large RF coils can couple inductively as well as capacitively to the cables even though the interfacing cables have filters on the control wires. Also ground loop currents through the shield jacket of the interfacing cables are not affected by the filters. Filters alone thus have had difficulty in dealing with the spurious RF energy.
These problems associated with induced RF currents in the ground loops through the shielding of the control lines are worsened by the high frequency at which the MRI devices operate. Due to the nature of the MRI process, the RF coils must transmit and receive RF signals of a predetermined frequency (the so-called "Larmor frequency," which will depend upon the magnitude of the magnetic fields employed and the magnetogyric ratio of the nuclei being imaged; the frequency is approximately 15 MHz in some machines). At these high frequencies, achieving a single point ground is impractical because the capacitive reactance of the shielded jacket to ground is low, even though the shield o may not be mechanically attached by a conductive wire to ground.
Techniques used to eliminate ground loop currents in multiple-ground systems can be found in the text book "Noise Reduction Techniques in Electronic Systems," H. W. Ott, John Wiley & Sons (1976). These techniques include the use of isolation transformers, the winding of coaxial cables around magnetic cores, and the use of optic couplers. The transformer and magnetic core techniques, however, are not suitable for RF coils because they include strongly magnetic materials that interfere with the static magnet used in the MRI device. These techniques would cause particular problems in MRI systems where one is attempting to generate homogeneous magnetic fields in the imaging area. The use of an optical coupler to interrupt the ground loop in an MRI device would require considerable engineering effort.
Other methods for reducing radio frequency interference caused by the ground loops between the control circuitry and the RF coils includes 1) double shielding the RF coaxial cables between the control device and the RF coils and 2) shielding the control circuitry in a shielding enclosure. These methods have proven to be relatively costly and are often ineffective.
Harrison et al., U.S. Pat. No. 4,682,125 (commonly assigned with the present application) discloses an RFI choke for use in the coaxial cables carrying the RF signals between the RF coils and the RF transmitter/receiver.
The Harrison et al. RFI choke assembly includes a coil of coaxial cable between the RF coil and the transmitter/receiver having a number of turns that provide a certain inductance and a lump fixed capacitance connected in parallel across the coiled section. Harrison et al also provides a tuning rod of brass, copper or aluminum positioned within the center of the coil section to achieve more precise parallel resonance. While Harrison o et al. used its RF ground breaker to interrupt the coil coupling through the coaxial cables between the RF coils and the transmitter/receiver, the Harrison et al. RF ground breakers do not break the ground loop through the shield jacket on the control lines between the RF coils and the control circuitry.